Serve Prometheus metrics at /metrics behind basic auth (closes #94)

With METRICS_USERNAME and METRICS_PASSWORD both set, the backend
records request metrics through go-http-metrics in a registry of its
own, with Go's runtime and process metrics, and serves them at
GET /metrics behind basic auth; nginx passes /metrics to it. With
neither set there is no such route; one alone, or a METRICS_USERNAME
containing ":", stops the start with an error naming the setting.

Only requests that reach the health check or POST /api/v1/reports are
recorded, as the labels are path and method, which clients could
otherwise make up without end; POST /api/v1/reports is registered by
its full path for that.

Deviation: go get and go mod tidy ran directly; no entrypoint added a Go
dependency yet (issue #45).

Model: opus-5-5

Dockerfile

@@ -1,6 +1,6 @@
 # The one image netwatch ships: nginx serves the built frontend and
-# passes /api/ and /.well-known/healthcheck to netwatch-server, the Go
-# backend, which runs in the same container on loopback only.
+# passes /api/, /.well-known/healthcheck and /metrics to netwatch-server,
+# the Go backend, which runs in the same container on loopback only.
 # bin/entrypoint.sh starts and watches both.
 
 # Lint stage — fast feedback on formatting and lint issues. The